Do not mark obviously unreachable blocks live when processing PHI nodes, and handle incomplete control dependences correctly.  This fixes:
Do not mark obviously unreachable blocks live when processing PHI nodes,
and handle incomplete control dependences correctly.  This fixes:

Regression/Transforms/ADCE/dead-phi-edge.ll
  -> a missed optimization

Regression/Transforms/ADCE/dead-phi-edge.ll
  -> a compiler crash distilled from QT4


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@20227 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,llvm-mirror/llvm,chubbymaggie/asap,chubbymaggie/asap,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,dslab-epfl/asap,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,chubbymaggie/asap,apple/swift-llvm,dslab-epfl/asap,apple/swift-llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,dslab-epfl/asap,dslab-epfl/asap,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,dslab-epfl/asap,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,chubbymaggie/asap,chubbymaggie/asap,chubbymaggie/asap,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm